Williams made a post on his personal blog this evening, discussing his plans in no great detail. The plans do, however, include staying at Twitter as a board member. Willimas wrote:

After stepping down from CEO six months ago, my mind started to wander. The reason I left Blogger/Google when I did is that I felt it had reached a place where it was on solid ground and in capable hands (at the time, Jason Goldman’s as product manager). Though still an independent company, I realized Twitter may be at a similar point today. So, as was reported in various places yesterday, I've decided to scale back my role at the company. (I'm still involved, but it's no longer my full-time job.)

I’m not ready to talk about what I have planned next, but I will venture a prediction about what's next for Twitter: It will be bigger and better.

...

First of all, I'm not disappearing from Twitter. I remain on the board of directors and will frequently meet with many folks there to help in any way I can.

However, now that Twitter is in capable hands that aren't mine, it’s time to pick up a whiteboard marker and think fresh. There are other problems/opportunities in the world that need attention, and there are other individuals I’d love to get the opportunity to work with and learn from. (Details to come.)